Semiconductor devices such as flash memory devices are often present in computers and other electronic products (e.g., digital television, digital cameras, digital audio players, and memory cellular phones) to store data and other information. Flash memory devices may also be used as portable storage devices, such as portable Universal Serial Bus (USB) flash drives or “thumb” drives. Flash memory devices may retain the information stored in the device even when power is disconnected from the device.
A flash memory device stores information in numerous memory cells, which are usually formed in a semiconductor chip. Each of the memory cells often has a metal-oxide semiconductor (MOS) transistor with two different transistor gates: a control gate and a so-called “floating” gate. The control gate may be used to turn the transistor on and off to control access to the memory cell. The floating gate may be used to store one or more bits in each memory cell.
The number of electrons in the floating gate may determine the value of the information stored in memory cell. Different numbers of electrons in the floating gate may correspond to different values of information. The number of electrons in the floating gate may be varied by either adding electrons to or extracting electrons from the floating gate. A flash memory device usually has a programming operation (which is also sometimes referred to as a write operation) to add electrons to the floating gate, and an erase operation to extract electrons from the floating gate. Information in the memory cells may be read in a read operation. Each memory cell has a threshold voltage value dependent on the number of electrons in the floating gate. A read operation determines the threshold voltage value of the memory cell being read to provide the value of the information stored in the memory cell.